The invention is directed to industrial process emission control for industrial waste gas containing formaldehyde. Specifically, the invention is directed to an adsorption, desorption-photochemical oxidation (ADPO) system for minimizing formaldehyde levels in the gaseous effluent.
Formaldehyde is a ubiquitous, hazardous pollutant emitted from a variety of large-scale industrial processes, including wood processing, furniture manufacturing, painting and coating, textile production, and cement manufacturing processes. Industrial processes emitting hazardous materials are regulated subject to reporting and mandated abatement requirements. Formaldehyde is a known carcinogen and subject to U.S. Environmental Protection Agency xe2x80x9cMaximum Achievable Control Technologyxe2x80x9d (MACT) standards. Large-scale plants that emit in excess of 10 tons of formaldehyde annually are classified as major sources under these MACT standards.
Incineration is a high-energy process and often leads to non-benign secondary emissions such as nitrogen oxides (NOx) and unburned hydrocarbons. Photocatalysis systems are relatively capital intensive to install and require high maintenance to avoid degrading efficiencies and treatment reliability. Other techniques lead to secondary wastes and leave the ultimate fate of the pollutants unresolved. A technique is needed that can reliably treat chemical pollutants in a cost-effective manner.
Known methods to reduce formaldehyde emissions include eliminating the sources from raw materials, incineration, activated carbon-bed adsorption, adsorption on zeolite supports, and aerogels and adsorption techniques in conjunction with photocatalytic destruction. Obvious drawbacks of incineration include the cost of fuel and additional treatment of CO and NOx emissions. Costs incurred in the use of activated carbon include disposal and/or recovery costs in handling spent activated carbon. Captured volatile organic compounds (VOC) are not destroyed as they are retained only by a physical adsorption mechanism since chemisorption mechanisms are not provided in the use of activated carbon.
The costs associated with the addition and recovery of exogenous photocatalysts introduced to a waste stream containing VOC are obvious. Avoidance of specially adapted substrates would be desirable especially in industrial processes with very high effluent volume and with relatively low concentrations of VOC. Also, catalytic methods involve exothermic reactions, and are sometimes accompanied by over oxidation from the favored attack of partially oxidized products over the starting VOC.
The current commercial technology for reducing SO2 and NOx emissions from power plants is wet scrubbing. Wet scrubbing generates hazardous slurry wastes. Dry sorbent injection technologies produce dry hazardous wastes. The dry sorbents are injected into a conventional power plant either in the combustion furnace (870-1200xc2x0 C.), the economizer outlet (300-400xc2x0 C.), or the air preheater outlet ducts (120-200xc2x0 C.). Typical sorbents include finely ground limestone, dolomite, hydrated lime, sodium bicarbonate, and sodium sesquicarbonate. Known use of dry sorbents reacted with SO2 emissions forming solid products, such as CaSO4 or Na2SO4, are then removed with the coal ash or with the dust by the particulate matter collection system, e.g., electrostatic precipitator or baghouse, depending on the point of injection.
Photochemical destruction of VOC is known. U.S. Pat. No. 3,977,952 discloses a process for the decomposition of one or more carbon-containing compounds such as in an industrial waste or flue gas containing VOC, oxygen and water vapor. The method is carried out by exposing humidified gas to radiation of a wavelength of about 20 to 600 nanometers.
In some industrial processes, such as pyroprocessing of cement, recovery of the particulate solids to the production produce is of economic importance. A discussion of dry sorption methods is found in U.S. Pat. No. 6,080,281 teaching an emission control process using photocatalytic and nonphotocatalytic aerogels for adsorption, and exposing the photocatalytic aerogel material containing adsorbed VOC to ultraviolet (UV) radiation resulting in their destruction.
U.S. Pat. No. 4,210,503 discloses a direct photolysis method for controlling gaseous emissions, and particularly vinyl chloride, by exposing the emissions to UV light and, thereafter, absorbing such decomposition products in a scrubber which substantially eliminates the vinyl chloride and most other decomposition products from the effluent stream.
U.S. Pat. No. 4,981,650 discloses a method to remove dioxin-contaminated waste by way of extraction in a liquid capable of extracting dioxins. A hydrogen donor is added to the extracting solvent or later on during addition of an activating agent. The dioxin-containing liquid extract is treated in a direct photolysis reactor which contains immersion UV lamps.
U.S. Pat. No. 5,045,88 discloses the removal of halogenated and non-halogenated volatile and non-volatile organic contaminants from a gaseous stream by mixing a gaseous oxygen bearing substance with the contaminated gaseous stream, contacting the mixture with a solid photocatalyst; and exposing the photocatalyst and organic components to UV light having a wavelength up to 600 nanometers. The catalyst is pre-selected to prevent formation of a liquid phase.
U.S. Pat. No. 5,417,825 discloses a thermal photolytic process that utilizes high temperatures in combination with radiation exposure to induce a photochemical reaction to detoxify a wide variety of organic pollutants, for example, chlorinated aromatic hydrocarbons. The hydrocarbons are treated in the gaseous phase by heating to a temperature greater than 200xc2x0 C., preferably 600-800xc2x0 C., and exposing the heated gas to radiation at wavelengths of less than 280 nanometers, preferably from 185 nanometers to 280 nanometers, for at least two seconds.
U.S. Pat. No. 5,650,549 teaches a photothermal process for the detoxification of chlorinated aromatic hydrocarbons contained in a gas stream comprising the steps of: heating chlorinated aromatic hydrocarbons to a temperature of greater than 200xc2x0 C. to form a gas stream or maintaining a pre-existing chlorinated aromatic hydrocarbon containing gas stream produced from a combustion source at a temperature of greater than 200 xc2x0 C. and exposing the gas stream to radiation at a wavelength of less than 280 nanometers for at least one second to convert said chlorinated aromatic hydrocarbons nontoxic reaction products, and releasing said gas stream to the atmosphere.
U.S. Pat. No. 5,839,078 discloses a method of direct vitrification of nuclear waste comprising the steps of providing waste in the form of relatively small pieces with vitrifiable material, providing high intensity light source of sufficient power to cause melting and subsequent vitrification of said waste; and, cooling and storing said vitrified material.
U.S. Pat. No. 5,342,582 discloses an apparatus for reprocessing special wastes of photopolymerizable scrap material to produce domestic waste, comprising a housing equipped with a feed hopper, at least one UV emitter arranged in the housing to irradiate and heat the scrap material, and a chopper arranged in the housing to comminute the scrap material. The photocrosslinkable and thermally crosslinkable scrap is composed of, for example, dry resist, solder resist, color proof films, screen printing films, and the like, which form special waste because of their reactive constituents.
U.S. Pat. No. 5,476,975 discloses a method for photodegradation of a solution of organic toxic chemicals recoverable from contaminated wood products by the use of a super-critical fluid, by exposing the extracted solution to UV, in the presence of a photosensitizer.
U.S. Pat. No. 5,935,525 discloses a pre-treatment system and an air treatment system for abatement of contaminated air that includes pollutants such as VOC, NOx, and/or CO. The air stream is treated using UV light under conditions that produce hydroxyls, peroxides, and other oxidants without the formation of ozone. These oxidants are also utilized in the activated air, with the activated water being formed as an aqueous solution (vapor) of the activated air. The pre-treatment system includes a quenching zone where activated water is misted into the air stream, followed by alternating reaction zones and depletion zones where activated air is added and then turbulently mixed with the air stream. The air treatment system includes a primary treatment tunnel, carbon bed system, activated air generator, and a sparger tank farm. Activated air produced by the generator as added into water while being exposed to UV light in the sparger tank farm. Then, as the contaminated air stream moves through various sequential chambers within the tunnel, it is subjected to the misted activated water, while being simultaneously exposed to UV radiation. Air exiting the tunnel is then further treated in the carbon bed system.
U.S. Pat. No. 6,179,971 discloses a two-step process for air purification comprising a photolytic step followed by a photocatalytic step, each of which entail radiation treatment to convert contaminants into less harmful products. The method provides a photolytic stage having a source of UV radiation; and a downstream photocatalytic stage using a photocatalyst and source of UV radiation.
U.S. Pat. No. 5,538,537 discloses a method of desulfurizing furnace flue gases laden with SO2 comprising: cooling the flue gases to a temperature near but above the dew point thereof; and flowing the cooled flue gases through a bed of granular cement stone sorbent prepared from a mixture of cement and water. The sorbent laden with pollutants from the flue gases can be further processed directly in an advantageous manner in a cement plant, for example by grinding it together with cement clinker or separately therefrom and thereby adding it as a component, for example as a gypsum component, to a cement which is to be produced, so that no disposal problems exist for sorbent laden with pollutant. With the addition of ashes or fly ashes from coal or fluosolids furnaces a particularly environmentally friendly means for disposal of these ashes can be achieved simultaneously if a sorbent laden with pollutants from the flue gases is further processed for the production of cement (together with cement clinker). Sorbent is produced, it is advantageous to use it with a grain size of  greater than 1 mm, preferably approximately 4 to 20 mm. A mixture of granulated cement stone and carbonaceous sorption material then forms the sorbent used according to the invention which is brought into contact with flue gases which are to be purified.
U.S. Pat. No. 4,634,583 discloses a method for the desulfurization of a calcium containing flue gas stream from a firing system such as a cement-making plant wherein at least partially deacidified, hot raw cement meal is added to the flue gas at selected points to adsorb the sulfur oxides to the calcium present in the gas. No additional adsorption agents, for example, activated carbon, pure calcium oxide, milk of lime, or the like are used. Raw cement meal having an adequately high proportion free calcium oxide is conveyed to the conduit of the exhaust gas to be desulfurized. The preferred method comprises suspending the deacidified raw cement meal in the flue gas in the form of a cloud of airborne dust, and thereafter separating the dust from the flue gas after the sulfur oxides have been bonded to the calcium.
U.S. Pat. No. 5,137,704 discloses a process for decreasing NOx content of exhaust gases from cement-burning kilns by an addition of ammonia and/or ammonia-containing substances to the hot exhaust gases, the exhaust gases are desulfurized at a temperature from 50xc2x0 to 100xc2x0 C. in a dry or semidry process by a mixture of raw cement powder and calcium hydroxide. The mixed solids that have been removed from the exhaust gas in a dry state in the desulfurizing stage are returned to the exhaust gas stream when it is a temperature from 850xc2x0 to 1,000xc2x0 C.
Treatment methods for pollutant bearing gas in a corona discharge device is a known method of removing the pollutants. A general review of this technique is provided in Puchkarev et al., xe2x80x9cToxic Gas Decomposition by Surface Discharge,xe2x80x9d Proceedings of the 1994 International Conf. on Plasma Science, Jun. 6-8, 1994, Santa Fe, N.Mex., paper No. 1E6, page 88. Corona discharge systems used for removal of mercury are disclosed in U.S. Pat. No. 5,591,412.
Injection of activated carbon in waste gas effluent is known. See U.S. Pat. Nos. 4,196,173; 4,889,698; 5,053,209; 5,607,496; and 5,672,323.
Copending U.S. application Ser. No. 09/847,476 filed May 2, 2001 now U.S. Pat. No. 6,541,677 discloses a process for detoxifying polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD-PCDF) contained in an industrial gas stream. In a Stage I treatment, irradiation of particulate matter-laden gas stream emitted from the industrial process as the gas passes through a particulate matter filtering device to destroy precursors of PCDD-PCDF. In a Stage II treatment, filtered gas from Stage I enters irradiating a filtered gas stream with light in spectral range of 290 to 500 nanometers, in the absence of absence of added photocatalyst results in the direct photolytic dechlorination of PCDD-PCDF in the vapor phase.
In a basic aspect, the invention provides a multi-stage method for reduction or removal of formaldehyde from an industrial waste gas effluent stream, comprising introducing finely divided particulate solids containing an organic component into a gaseous effluent stream, adsorbing formaldehyde onto the solids, separation of the gas from the particulate solids, contacting the particulate solids in a hot gas desorption step using a separate gas stream, irradiating the hot gas containing desorbed formaldehyde and the effluent gas stream to convert formaldehyde to less hazardous chemical species.
In another aspect, the present invention involves irradiation of an in-line gaseous formaldehyde-containing effluent after having been mixed with particulate adsorbent solids and the adsorbent has been removed. In this case, the irradiation destroys formaldehyde that was not adsorbed on the adsorbent. The irradiation employed has a frequency in the UV range and is readily adjusted in accord with the teaching herein. The treatment of formaldehyde with radiation in accordance with the invention is especially efficient when employing UV sources that emit in the spectral range of 220 to 370 nanometers under conditions typically encountered in industrial effluent gases containing less than 100 ppm of formaldehyde. The preferred intensity and frequency of the radiation taught herein results in the rapid and efficient direct photolytic destruction of formaldehyde prior to release of the effluent gas stream to the atmosphere.
In a general component aspect, an industrial production process which emits a gaseous effluent stream containing formaldehyde is coupled to a Stage I formaldehyde reduction method, the method comprising:
a) introduction of solid particulates into a gaseous effluent gas stream from the process to form a gas-solid mixture, the solid particulates containing an organic component for the adsorption of formaldehyde;
b) recovering solid particulate matter with adsorbed formaldehyde from the gas-solid mixture;
c) generating desorbed recovered solid particulates by desorbing formaldehyde into a second gas stream;
d) oxidizing formaldehyde in the second gas stream containing desorbed formaldehyde by direct photochemical oxidation in the absence of added photocatalyst by irradiating the gas stream with UV light; and
e) returning desorbed solid particulates to said process and/or the removal method.
In accordance with another aspect of the invention in an industrial production process that emits a gaseous effluent stream containing formaldehyde, a Stage II formaldehyde reduction method is provided, the method comprising:
a) introduction of solid particulates into a gaseous effluent stream from said process to form a gas-solid mixture, said solid particulates containing an organic component which adsorbs formaldehyde;
b) forming a filtered gas stream from the separation of solid particulates from said gas-solid mixture; and
c) oxidizing formaldehyde in said filtered gas stream by direct photochemical oxidation of formaldehyde in the absence of added photocatalyst by irradiating the filtered gas stream with ultraviolet light.
In accordance with a third aspect of the invention there is provided a coupled Stage I and II (S-I, S-II) process for the removal of formaldehyde from an industrial process having a gaseous effluent stream which contains formaldehyde, a removal method comprising:
a) introduction of solid particulates into a first effluent gas stream to form a gas-solid mixture, said solid particulates containing an organic component which adsorbs formaldehyde from the effluent gas stream;
b) separating said gas-solid mixture into recovered solid particulates and a filtered first gas stream;
c) oxidizing formaldehyde in said filtered first gas stream by direct photochemical oxidation in the absence of added photocatalyst, (S-II) by irradiating the filtered first gas stream with ultraviolet light to form an irradiated first filtered gas stream;
d) contacting a heated second gas stream with said recovered particulates thereby desorbing formaldehyde into the heated second gas stream; and
e) oxidizing formaldehyde in said second gas stream by direct photochemical oxidation, in the absence of added photocatalyst (S-I) by irradiating the second gas stream with UV light, to form an irradiated second gas stream.
In a fourth aspect, the invention resides in a method to remove formaldehyde from an effluent gas emitted from an industrial process which also emits finely divided byproduct solids, the method comprising introducing the byproduct solids into the effluent gas stream, adsorbing formaldehyde on to the solids, recovering the solids from the gas stream by a particulate matter control device, desorbing formaldehyde from the solids into a hot gas stream, irradiating the hot gas stream to reduce the formaldehyde content, and recovering the solids to the industrial process.
In a fifth aspect, the invention includes a method for pryoprocessing of cement raw feed comprising a pyroprocessing step including directing raw feed to a kiln to produce cement clinker, directing the pyroprocessing gas effluent into a mixing zone, mixing the gas with finely divided particulate solids previously recovered from pyroprocessing, adsorbing formaldehyde onto the finely divided solids, desorbing formaldehyde in a desorption zone by contactng with a flow of hot gas, irradiating the hot gas reducing the level of formaldehyde therein, and irradiating the filtered pyroprocessing gas effluent reducing the level of formaldehyde therein, and returning the solids or the treated gas to the industrial process.
Various specific and/or preferred aspects are specified hereinbelow, and other objects and advantages of the present invention will become apparent and obvious from a study of the following description and the accompanying drawings that are merely illustrative of such invention,